Toothed belt transmissions each having a toothed belt and two toothed pulleys for transmitting power are known from Japanese Patent Laid-Open Publication No. HEI-11-132291 entitled “TOOTHED BELT TRANSMISSION DEVICE” and Japanese Patent No. 2995021 entitled “SYNCHRONOUS PULLEYS AND POWER TRANSMISSION USING THE SAME”.
In the toothed belt transmission of HEI-11-132291, a toothed belt has tooth edges and tooth bottoms formed by respective arcs and tooth edges and tooth bottoms of sprockets are formed by respective arcs. With this arrangement, the tooth bottoms of the toothed belt and the tooth edges of the sprockets are brought into tight engagement with each other such that relative slippage between the tooth bottoms and the tooth edges is limited to preclude deterioration in a wear-resistant property.
In the transmission of Japanese Patent No. 2995021, a toothed pulley includes tooth grooves each defined by a pair of concave or arcuate pressure-bearing surfaces provided on opposite sides of a center line of a tooth groove. Curvature centers of the arcs forming the pressure-bearing surfaces are positioned on a straight line intersecting the groove center line. A distance (on the groove center line) between the straight line and a groove bottom surface is set to cause the curvature center to be positioned outside a pulley pitch line so that a peripheral dimension of each tooth groove becomes smaller than that of a conventional tooth groove in which the curvature center lies on the pulley pitch line, resulting in a greater peripheral length of a pulley land portion than that of the conventional pulley land portion. As a result, an increase in a surface pressure between the pulley land and a belt land of the toothed belt is inhibited to thereby preclude wear of the belt caused by the increased surface pressure, thus improving durability of the toothed belt.
Both transmissions of Japanese Patent Laid-Open Publication No. HEI-11-132291 and Japanese Patent No. 2995021 are directed to technologies in which by improving the tooth portion or the shape of the tooth grooves of the toothed belt and the toothed pulley (or sproket), durability of the toothed belt transmission is improved. For example, when applying the toothed belt transmission to an engine of small outer dimension, the toothed belt is trained around the toothed pulley (or sprocket) of small diameter and the belt per se, especially, its tooth root, is caused to locally bend at a large extent. Further, with the toothed belt in which an endless core wire is buried along a peripheral length of the toothed belt in order to prevent extension, the core wire is caused to locally bend at the large extent. This results in degradation in durability of the tooth portion or the core wire of the toothed belt.
To this end, it is desired to provide a toothed belt transmission having a toothed pulley with a small diameter in mesh with a toothed belt while the toothed belt has an improved durability.